3GPP Long Term Evolution, LTE, is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project, 3GPP, to improve the Universal Mobile Telecommunication System, UMTS, standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network, UTRAN, is the radio access network of a UMTS and Evolved UTRAN, E-UTRAN, is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a User Equipment, UE, is wirelessly connected to a Radio Base Station, RBS, commonly referred to as a NodeB, NB, in UMTS, and as an evolved NodeB, eNodeB or eNodeB, in LTE. An RBS is a general term for a network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE.
A currently popular vision of the future development of the communication in cellular networks comprises large numbers of small autonomous devices, which typically transmit and receive only small amounts of data infrequently, for instance once per week to once per minute. These devices are generally assumed not to be associated with humans, but are rather sensors or actuators of different kinds, which communicate with application servers for the purpose of configuration of and data receipt from said autonomous devices within or outside the cellular network. Hence, this type of communication is often referred to as machine-to-machine, M2M, communication and the devices are denoted Machine Devices, MDs. The nomenclature used in 3GPP standardization for the communication is Machine Type Communication, MTC, whereas the devices are denoted MTC devices. As these devices are assumed to typically transmit rather seldom, their transmissions will in most cases be preceded by a Random Access, RA, procedure, which establishes the device's access to a network and reveals the device's identity to the network.
Internet of Things (IoT) and the related concept of Machine-Type Communication (MTC) is an important revenue stream for operators and have a huge potential from the operator perspective. It is efficient for operators to be able to serve MTC UEs using already deployed radio access technology. Therefore, 3GPP LTE has been investigated as a competitive radio access technology for efficient support of MTC 3GPP TR 36.888 v12.0.0. Lowering the cost of MTC UEs is an important enabler for implementation of the IoT. Many MTC applications will require low operational UE power consumption and are expected to communicate with infrequent, bursty transmissions and small-size data packets. In addition, there is a substantial market for the M2M use cases of devices deployed deep inside buildings which would require coverage enhancement in comparison to the defined LTE cell coverage footprint.
3GPP LTE Rel-12 has defined a UE power saving mode allowing long battery lifetime and a new UE category allowing reduced modem complexity. In Rel-13, further MTC work is expected to further reduce UE cost and provide coverage enhancement. The key element to enable cost reduction is to introduce reduced UE bandwidth of 1.4 MHz in downlink and uplink within any system bandwidth.
In LTE the system bandwidth can be up to 20 MHz and this total bandwidth is divided into physical resource blocks (PRBs) of 180 kHz. The low-complexity UEs with reduced UE bandwidth of 1.4 MHz that will be introduced in LTE Rel-13 will only be able to receive a part of the total system bandwidth at a time—a part corresponding to up to 6 Physical Resource Blocks, PRBs, in a subframe. In the following, we refer to a group of 6 PRBs as a ‘PRB group’ or a ‘narrowband’.
In 3GPP, coverage enhancement is proposed for MTC applications. In order to achieve the coverage targeted in LTE Rel-13 for low-complexity wireless devices and other types of wireless devices operating delay tolerant MTC applications, time repetition techniques may be used, i.e., enabling energy accumulation of the received signals at the network node, also known as eNB, to achieve such coverage enhancements. For physical data channels (PDSCH, PUSCH), subframe bundling (a.k.a. TTI bundling) can be used. When subframe bundling is applied, each HARQ (re)transmission consists of a bundle of multiple subframes instead of just a single subframe. Repetition over multiple subframes can also be applied to physical control channels. Depending on a UE's coverage situation, different number of repetitions will be used.
From the physical layer perspective, the random access procedure encompasses the transmission of a random access message, also known as a random access preamble, and random access response. A physical random access channel, PRACH, occupies 6 resource blocks in an uplink subframe or in a set of consecutive uplink subframes reserved for random access message transmissions. In the context of the present disclosure, a random access attempt may be composed of multiple repetitions of random access message transmission. The number of repetitions in a random access attempt is also known as a repetition level. The repetition level is correlated to energy accumulation in the receiving eNB.
The maximum bandwidth that Rel-13 low-complexity wireless devices can read in any system is 6 Physical Resource Blocks, PRBs, in a subframe. Furthermore, Rel-13 low-complexity wireless devices often need multiple repetitions to transmit a random access attempt. Consequently, while coverage enhancement through random access message repetition has been proposed, solutions suitable or applicable for low-complexity wireless devices are still wanting.
Hence, there is a need to provide a random access procedure which provides sufficient coverage and is suitable for low-complexity wireless devices, such as low rate MTC devices.